1. Field of the Invention
The present invention relates to a white balance correcting device for use in a color video camera, and more particularly, to a device which obtains a video color balance with good precision when high luminance signals are inputted thereto.
2. Description of the Prior Art
First, a conventional white balance correcting device of a color video camera will be explained with reference to FIG. 1 showing a circuit block diagram thereof. In FIG. 1, an image output signal from an image pickup tube or a solid-state image element is supplied to a color separation circuit 4 through a pre-amplifier 1 and an AGC circuit 2 and a band-pass filter 3. The image output signal is divided by the separation circuit 4 into two color signals, i.e., an R signal and a B signal, which are supplied to an R gain-control matrix circuit 7 and a B gain-control matrix circuit 9. Also, a part of the output of the AGC circuit 2, i.e., only the low frequency component of the luminance signal thereof, is fed to the R gain-control matrix circuit 7 and B gain-control matrix circuit 9 through an iris gamma correction circuit 5 and a low-pass filter 6. In addition, a part of the output of the iris gamma correction circuit 5 is fed to a composite video signal framing circuit 13 through a low-pass filter 10. Then, an (R-Y) signal which is an output from the R gain-control matrix circuit 7 is supplied to an inverting input terminal of an operational amplifier 16 after being changed to a DC voltage by a smoothing circuit 14. On the other hand, a voltage to be referred to as a standard value in a reference image condition is supplied to a non-inverting input terminal of the operational amplifier 16. The output of the amplifier 16 is fed back to the R gain-control matrix circuit 7. Also, a (B-Y) signal which is output from the B gain-control matrix circuit 9 is supplied to an inverting input terminal of the operational amplifier 17 after being changed to a DC voltage by a smoothing circuit 15, and a reference voltage 27 in a reference image condition is supplied to a non-inverting input terminal of the operational amplifier 17. The output of the operational amplifier 17 is fed back to the B gain-control matrix circuit 9.
Assuming that a color temperature in a surrounding state becomes higher than that in the reference image condition, the voltage at the inverting input terminal of the operational amplifier 16 is lowered due to a reduction of the R component of the (R-Y) signal, resulting in the output voltage of the operational amplifier 16 becoming higher. The variation of this output voltage is fed back to the R gain-control matrix circuit 7 to control the R gain-control matrix circuit 7 so as to make the R component of the (R-Y) signal larger. On the other hand, since the B component of (B-Y) signal becomes larger when the color temperature becomes higher, the voltage at the inverting input terminal of the operational amplifier 17 becomes higher so as to make th output voltage of the operational amplifier 17 become lower. The variation of this output voltage is fed back to the B gain-control matrix circuit 9 to control the B gain-control matrix circuit so as to make the B component of the (B-Y) signal smaller.
However, with the above arrangement, if the luminance signal level becomes higher than a given level in the case of taking a picture at a condition of luminance which is higher than the reference signal, both the R signal component and the B signal component begin to be smaller with respect to the luminance signal level so as to reduce both the R component and B component of the (R-Y) signal and the (B-Y) signal, resulting in the output voltages of both the operational amplifiers 16 and 17 to become higher. The variations of these output voltages are fed back to both the R gain-control matrix circuit 7 and the B gain-control matrix circuit 9 to control both the R gain-control matrix circuit 7 and the B gain-control matrix circuit 9 so as to respectively make both the R signal component and the B signal component larger. Therefore, the conventional device has a disadvantage in that, within a modulator 8, and a semicircular filter circuit 18 and a composite video signal framing circuit 13, the range of values among R, G, and B components become abnormal so as to cause the whole picture to be changed to an image having a large amount of red.